L0301P56 - Development of the Nervous System
Nervous System Central Nervous System (CNS) *brain lies inside cranial cavity *spinal cord lies inside the vertebral canal *retina - in the eye *axial section of the nervous system Peripheral Nervous System (PNS) *12 cranial nerves *31 pairs of spinal nerves rising from spinal cord segments **8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal Gastrulation *begins in week 3 of human gestation *trilaminar germ disc formed **epiblast becomes: ***ectoderm, mesoderm, endoderm **hypoblast is displaced Notochord *induces differentiation of ectoderm into neural ectoderm *provides rigidity and integrity to the primitive organism (like a beam in the mass of cells) *provides bi-laterality - defined which sections were to become the left or right side of the organism Formation of the Notochord *specific regions of the epiblast migrate **to the cranial-most part of the node = notochord (n) ***cell intercalate with endoderm then detach to form the definitive notochord Extra: *next in cranial aspect = paraxial mesoderm (pm) *next portion = intermediate modem (im) *more caudal part of the streak = lateral plate mesoderm (lpm) *most caudal part = extraembryonic (chorionic) membrane (eem) Neurulation *ectoderm: **induced by the notochord **folds in on itself to form the neural groove ***section that has folded in = neural plate **neural groove edges = neural folds **neural folds come together at one point and zips up in both directions to form the neural tube ***when this does not occur properly, i.e. anterior or posterior neuropore does not completely zip up = spina bifida *paraxial mesoderm: **induced by formation of rat neural tube **forms segmental blocks called somites on either side of the neural tube Neural Crest Cells *form at the tips of the neural folds *delaminates from the neural tube when until the neural tube closure is complete *become: **dorsal root ganglions **sympathetic ganglions **parasympathetic ganglions **Schwann cells **melanocytes **adrenal medulla Growth of Motor Neurons *neural tube differentiates into: **alar plate - top **basal plate - bottom *motor neurons appear in basal plate *axons grow out towards particular somites as they continually migrate, by following chemical signals from them Growth of Sensory Neurons *arise from neural crest cells *situated in dorsal root ganglion *surrounded by satellite cells (glial) *pseudounipolar in shape **i.e. axon runs int two directions: ***central process ****grows towards the dorsal horn of the spinal cord ***peripheral process ****grows out to body walls and limbs *****skin, muscles, joints Lateral Plate Mesoderm *forms bones, tendons, ligament and connective tissue in limbs Spinal Cord Development *spinal cord forms *subdivides into functional ventral and dorsal zones Brain Development *forms as a series of swellings **fore, mid and hindbrain *brain stem forms alar and basal plate similar to the spinal cord Somites *somites are found in all vertebrates *somite splits into the dispersing cells: **sclerotome ***from the ventral and medial walls of the somite ***gives rise to: ****the connective tissue and cartilage ****vertebrae and ribs (bones of the axial skeleton) *****in an segmental way, i.e. adjacent sclerotomes supply cells each half *dermo-myotome **from dorsomedial and ventrolateral regions of the somite **splits into: ***dermatome ****form the deep layers of the dermis ***myotome ****forms the muscles *****dorsal: epimere - post vertebral musculature *****ventral: hypomere - muscles of the body wall and limbs